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How Increased System Gain Relates to Range

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Article Number000001678
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When calculating Fade Margin or performing a Link Budget Analysis, the total system gains and losses can indicate how dB relates to achievable distance. Receiver sensitivity, power output, antenna gain, and antenna cable attenuation, coupled with Fresnel Zone considerations (line of sight conditions) are all factors that will influence the range an RF network can achieve.

The user may have limited, or no control over many of these factors and may need high gain antennas to increase the overall gain of a system. But just how much distance will be gained from an increase in antenna gain? Again, there are many factors that can influence the answer, antenna height, the environment, directional vs. omni-directional antenna, etc.

Most of these factors are calculated as part of the Friss Transmission Equation. The Friss Equation is discussed in further detail in the article: Receiver Sensitivity and Blocking Specifications. To simplify, a few rules of thumb can be utilized to gage the approximate contribution of increased antenna gain.

12dB of increased system gain is necessary to double the transmission distance in typical non RF line of sight conditions.

6dB of increased system gain is necessary to double the transmission distance in typical RF line of sight conditions.

The necessary gain can be achieved through an increase in gain from one, or both ends of a wireless link. For example, Digi radios are typically shipped with 2.1dBi omni-directional dipole antennas. Removing the dipole antenna on one end of the system will decrease the system gain by 2.1dB. If the antenna were replaced by a high gain omni-directional antenna with a gain of 14dBi, the overall system gain would have increased by 12dB, which would have approximately doubled the range of that system.

Instead, if the dipoles on both sides of the system were removed, and replaced with 8.1dBi gain omni-directional antennas (a more feasible option), the entire system gain would have increased by the necessary 12dB, and the range would again approximately double.

There are some considerations when using the rules of thumb and high gain antennas. For instance, high gain antennas deployed indoors are not nearly as effective as when deployed outdoors. Replacing an omni-directional antenna with a directional Yagi antenna will not always result in an improved RF link and in some instances could actually make it worse.

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